1. Field of the Invention
The present invention relates generally to a combustor and more specifically to a combustor which is appropriate for use as a gas turbine combustor.
2. Description of the Prior Art
One example of a premixed flame type combustor which is used as a prior art gas turbine low NOx type combustor is shown in FIG. 13, wherein FIG. 13(a) is a longitudinal cross sectional view of the combustor and FIG. 13(b) is a cross sectional view taken on line C--C of FIG. 13(a).
In FIG. 13, within a combustor 201 which is provided for a gas turbine cylinder, there are provided a plurality (eight pieces in this case) of main burners 202 around a central axis of the combustor 201. Each of the main burners 202 comprises therein a main fuel nozzle 203 of the same shape for all the main burners 202, and a main swirler 204 of also the same shape. Also, provided in a portion surrounded by the plurality of main burners 202 and along the central axis of the combustor 201 is a pilot burner 207, which comprises therein a pilot fuel nozzle 206 and a pilot swirler 205 provided around the pilot fuel nozzle 206.
Combustion air flowing inside of an outer periphery of the combustor 201 turns at an angle of 180.degree. at an air inflow portion 208 to flow in the combustor 201 passing through the main swirler 204 of each of the main burners 202 and the pilot swirler 205 of the pilot burner 207.
In the pilot burner 205, pilot fuel supplied from the pilot fuel nozzle 206 is burned by the combustion air which has passed through the pilot swirler 205. In the main burner 202, main fuel supplied from the main fuel nozzle 203 and the combustion air which has passed through the main swirler 204 are mixed to form a premixture, which is fired by a pilot flame of the pilot fuel so that a low NOx combustion is effected in the combustor 201.
The premixture formed in the main burner 202 is fired by the pilot flame of the pilot fuel, as mentioned above. In this case, as there is substantially a regularity in a mixing state of the premixture between each of the plurality of main burners 202, the combustion state in each of the main burners 202 becomes regular. This results in a regularity in heat generation distribution throughout the combustor 201 along the central axis direction thereof, and there occurs a constant area where a large heat generation in the combustor 201 is concentrated.
For this reason, vibratory combustion is prone to occur due to such concentrated heat generation to cause a non-stability of the combustion, which results in a problem in that a low NOx combustion is hampered.
FIG. 14 is a cross sectional view showing one example of a prior art pilot nozzle of gas turbine. In FIG. 14, numeral 301 designates a nozzle body and numeral 302 designates an air passage in a peripheral portion of the nozzle body 301, into which air 311 is taken. Numeral 303 designates an oil fuel supply pipe which is provided in a central portion of the nozzle body 301 for leading therethrough an oil fuel 310. Numeral 304 designates a gas fuel passage for leading therethrough a gas fuel 312 when such is used. Numeral 305 designates an oil fuel injection port, numeral 306 designates an air injection port and numeral 307 designates a gas fuel injection port. In the nozzle so constructed, both the oil fuel 310 and the gas fuel 312 are usable wherein the fuel is injected from a tip end of the nozzle and the air 311 for diffusion and water 309 for cooling are injected as described later so that combustion is effected.
FIG. 15 is an enlarged cross sectional view of a tip end portion of the pilot nozzle of FIG. 14. In FIG. 15, when an oil fuel is used, the oil fuel 310 is supplied through the oil fuel supply pipe 303 to be injected for combustion into a combustion chamber from the oil fuel injection port 305 of the central portion. On the other hand, the air 311 flows through the air passage 302 in the peripheral portion of the nozzle body 301 to be injected from the air injection port 306 for diffusion of the fuel. Also, the water 309 is injected from a water injection port 308 provided around the oil fuel injection port 305 to cool a peripheral portion of the oil fuel injection port 305.
In the above-mentioned pilot nozzle, the oil fuel 310 injected from the oil fuel injection port 305 spreads into the surrounding area, as shown in FIG. 15. On the other hand, the air 311 is injected from the air injection port 306 at such an angle as to cross the oil fuel 310 so spreading, thus there is formed therebetween a stagnation area 320 into which neither the air 311 nor the oil fuel 310 comes. But a mist 321 is formed by a portion of the oil fuel 310 scattering there, and this mist flows into this stagnation area 320 and sticks to the tip end of the nozzle to accumulate there as an unburnt carbon 322. This unburnt carbon 322 increases gradually so that the water injection port 308 may be plugged and the flow of the air 311 from the air injection port 306 may be obstructed. As a result, there arises a problem in that the cooling performance of the peripheral portion of the oil fuel injection port 305 is deteriorated or the combustion performance is badly affected.
In case the oil fuel is used in the prior art gas turbine pilot nozzle, as mentioned above, the stagnation area 320 is formed between the oil fuel injection port 305 and the air injection port 306 and the mist 321 of the oil fuel 310 scatters and sticks to the tip end of the nozzle to accumulate while being carbonized as the unburnt carbon 322. This results in plugging of the water injection port 308 to obstruct the injection of the water which causes a problem affecting the nozzle performance such that the cooling performance of the peripheral portion of the oil fuel injection port 305 is deteriorated or the flow of the injected air is obstructed.